1. Field
The present disclosure relates to MIMO (Multiple-Input, Multiple-Output) communication methods, and, more particularly, to a MIMO communication method of transmitting and receiving a pilot signal.
2. Description of the Related Art
In current years, with the widespread use of smartphones, the amount of wireless communication performed by mobile terminals has explosively increased. It is expected that communication between devices without human intervention will be performed in the future. From 2020 onward, a traffic volume is likely to increase by a factor of 1000 to 10000. Therefore, a new communication system is actively being studied. That is, in addition to LTE and LTE-A that have already been commercially practical, a system with which higher frequency utilization efficiency is achieved is being studied and is expected to be proposed as the fifth generation system of the 3rd Generation Partnership Project (3GPP).
Massive MIMO is one of effective technologies.
As illustrated in FIG. 10, in a communication system, a base station BS has many (Nb) transmission antennas (Ant#1 to Ant#Nb) and communicates with a plurality of (N) terminals (pieces of User Equipment: UE#1 to UE#N) at the same time via channels h00 to h0(N-1) to channels h(Nb-1) 0 to h(Nb-1)(N-1). The number of transmission antennas (Nb) in the base station BS is, for example, approximately 100, and the number of terminals is, for example, approximately 10.
With this structure, frequency utilization efficiency that has been affected by communication path noise and rapidly changing fading can be stably obtained almost without such effects. As compared with MIMO in the related art, a communication path capacity markedly increases. From these reasons, the massive MIMO is the most promising next-generation technology.
The massive MIMO system was introduced in a paper around 2010 and has been studied for its practical use. The results of the study have been actively reported since the middle of 2013. Detailed examples of the results of the study are, for example, Non-Patent Literatures 1 and 2. Referring to these literatures, the most important part of the study is channel estimation that is performed with pilot signals. For example, when the number of antennas of a base station is 100 and the number of terminals (each having one antenna) is 10, MIMO channels become a 10×100 matrix. This state is quite different from that in a MIMO system in the related art. More accurate channel estimation is required.
In a massive MIMO data section, terminals simultaneously perform transmission operations that overlap one another. The transmitted signals are received by many receiving antennas of the base station BS. The autocorrelation of the received signals is performed for channel diagonalization. The channel diagonalization allows the terminals to be distinguished one another. When the channel diagonalization is incompletely performed, interference among the terminals occurs and communication fails. For accurate diagonalization, more accurate channel estimation is required.
A channel estimation method that is currently the mainstream is performed with pilots that are transmitted from terminals and received by a base station. In this method, in order to avoid interference among the pilots transmitted from the terminals, it is assumed that the pilots are transmitted in slots that do not overlap on a time-frequency axis.
This method will be described with reference to FIG. 9. In the present disclosure, as a method of distinguishing between an uplink (transmission from a terminal to a base station, Up Link: UL) and a downlink (transmission from the base station to the terminal, Down Link: DL), time division duplex (TDD) is employed. In the TDD method, the uplink and the downlink are alternately arranged on a time axis so as to prevent a situation in which different channels are used for the uplink and the down link and to flexibly adapt to the change of a traffic volume. For the simplification of the drawing, eight terminals are illustrated in FIG. 9.
A signal stream includes headers preceding data slots both in uplink and downlink. When pilot signals reach a base station, a delay occurs because of multipath. Assuming that the maximum delay time is Tg, the number of pilots that do not mutually interfere with one another in a single TDD slot Ts is Ts/Tg. Referring to FIG. 9, four pilots are included in a single TDD slot Ts. Since all pilots for eight terminals cannot be included in this state, the transmission of pilots is also performed with the next TDD slot. While a certain terminal transmits a pilot, the other terminals cannot perform transmission. In the drawing, this term is illustrated as NA (Not Available). Thus, a pilot period increases a total overhead and degrades frequency utilization efficiency and throughput.
A result of a single channel estimation is maintained on condition that a terminal is not moved. However, when the terminal moves and a channel variation occurs, diagonalization cannot be performed. Accordingly, it is necessary to perform channel update at regular time intervals. This time interval is referred to as coherence time. The base station BS needs to have more accurate channel information at all times. It is therefore desirable that the base station BS perform channel update in the shortest possible time. On the other hand, however, the channel update performed on a terminal that is not being moved causes an increase in overhead.
Furthermore, many control signals are transmitted between the base station BS and the user equipment UE. Since the transmission of these control signals is also performed with headers, when headers are occupied with pilots and control signals, overhead may increase and a total throughput may decrease.